Field of the Invention
The disclosure relates to a resonant sensor.
Priority is claimed on Japanese Patent Application No. 2014-173987, filed Aug. 28, 2014, the contents of which are incorporated herein by reference.
Description of Related Art
A resonant sensor has a weight having predetermined weight, a spring holding the weight, a damping member disposed near the weight, and a resonator embedded in the spring. For example, the resonant sensor measures acceleration by detecting a change of resonant frequency of the resonator caused by strain of the spring. The strain of the spring is generated in proportion to the acceleration. A detection of resonant frequency of the resonator, which includes resonant frequency changed by the strain, is performed by vibrating the resonator by using an exciting circuit and detecting the resonant frequency.
A natural frequency of the resonant sensor is determined in accordance with a weight of the weight and a spring constant of the spring. A damping characteristic of a frequency characteristic of the resonant sensor is changed in accordance with a size of a gap formed between the weight and the damping member and pressure in the gap. For the reason, if the size of the gap and the pressure in the gap are adjusted, the resonant sensor having a desired frequency characteristic according to the purpose of use can be implemented. The gap acts as a damper with respect to the weight.
The size of the gap and the pressure in the gap are adjusted so that the frequency characteristic of the resonant sensor becomes a characteristic of critical damping. On the other hand, the resonator embedded in the spring is vacuum-sealed so as to achieve a high Q value. Therefore, the resonant sensor is designed so that the pressure around the weight is different from the pressure around the resonator.
In the resonant sensor, in a frequency range lower than the natural frequency, the strain of the spring is generated in proportion to acceleration. In a frequency range around the natural frequency, the strain of the spring is generated in proportion to velocity. In a frequency range higher than the natural frequency, the strain of the spring is generated in proportion to displacement. Therefore, in addition to acceleration, the resonant sensor can measure jerk, velocity, displacement, and so on.
In Japanese Examined Patent Application Publication No. H7-6852, U.S. Pat. No. 5,090,254, Japanese Patent No. 3,223,358, and D. W. Burns et al., “Sealed-cavity resonant microbeam accelerometer”, Sensors and Actuators A, Vol. 53, 1996, p. 249-255, a resonator which is the same as the resonator disposed in the resonant sensor is disclosed. In Japanese Patent No. 3,544,979, an accelerometer using resonant beam is disclosed. In Japanese Patent No. 5,158,160 and Japanese Patent No. 5,429,696, a resonant transducer used for measuring pressure is disclosed.
In recent years, from a perspective of improving measurement accuracy, it is required to improve dynamic range of the resonant sensor. So as to improve the dynamic range of the resonant sensor, a stiffness of the spring is made lower (the spring is made soft), and the weight is made heavier (the displacement is larger with respect to input). Thereby, the strain of the spring is easily generated by an input acceleration. Therefore, the resonant sensor can be designed so that the strain (tensile strain and compression strain) added to the resonator becomes larger.
Even if the tensile strain becomes larger, creep or destruction of the resonator does not easily occur. However, if the compression strain becomes larger, the resonator is easily buckled. For example, the value of the tensile strain, at which the creep or the destruction of the resonator is generated, is approximately from one thousand [ppm] to several tens of thousands [ppm]. On the other hand, the value (absolute value) of the compression strain, at which the resonator is buckled, is approximately from several tens [ppm] to several hundred [ppm]. In this way, if the compression strain which is approximately from a hundredth to a thousandth of the tensile strain, at which the creep or the destruction of the resonator is generated, is added to the resonator, the resonator is buckled. Therefore, although the dynamic range of the input acceleration (positive input acceleration) which causes the tensile strain of the resonator can be expanded, it is difficult that the dynamic range of the input acceleration (negative input acceleration) which causes the compression strain of the resonator is expanded.
So as to improve the dynamic range of the resonant sensor, if the stiffness of the spring is made lower, or if the weight is made heavier and the strain added to the resonator becomes larger, amount of the change of the resonant frequency also becomes larger. Therefore, the resonant frequency of the resonator may be the same as the resonant frequency (including high-order mode) of the spring.
In a case that such situation occurs, if a bending direction of the spring is the same as a vibrating direction of the resonator, energy of the resonator is absorbed by the spring. As described above, the detection of resonant frequency of the resonator is performed by vibrating the resonator by using the exciting circuit and detecting the resonant frequency. However, in a case that such situation occurs, the energy for vibrating the resonator is absorbed by the spring, as a result, there is a problem that measurement accuracy is significantly worsened.